Integrated analysis of structures and landforms of an intermontane longitudinal valley (Pinjaur dun) and its associated mountain fronts in the NW Himalaya

Abstract

Frontal regions of the Himalaya associated with topographic recesses are commonly marked by the development of intermontane valleys, locally referred to as “duns.” The relationship between structural evolution and landform development of an intermontane longitudinal valley (Pinjaur dun) is explored to understand the role of tectonic processes on landform development. The Pinjaur dun has a length of ˜ 180 km; it is divisible into a wider (˜ 20 km) northwestern and a narrower (˜ 6-8 km) southeastern segment. The Barsar thrust, the Nalagarh thrust, and the Himalayan frontal thrust form the mountain fronts in the area. The proposed structural model integrates the transverse structures (for e.g., Ghaggar tear fault) with the sequential development of imbricates in front of the Main Boundary thrust. The Pinjaur dun formed as a consequence of the fold-thrust-related uplift that formed the outermost Siwalik hills. Most of the geomorphic units formed within the dun are the result of interactions between the imbricates, such as the Pinjaur thrust, Jhajara thrust, and Pinjaur Garden fault. These interactions also resulted in the development of a significant and prominent drainage divide between the Jhajjara and the Sirsa Rivers of the Pinjaur dun. Landform development and landscape evolution in this frontal region of the Himalaya are a consequence of the interaction of thrust-fold and transverse fault zone tectonics; and constitute a record of transient (10²-< 10⁵ years) landscapes in association with deforming orogenic fronts.